INVERTEBRATES. 327 



the anterior to the posterior margin, 0.51 inch; length of hinge 

 0.46 inch ; convexity of the left valve about 0.05 inch. 



In some of its varieties this species seems to approach quite nearly Aviculo- 

 pecten recti-laterarius, Cox, sp., but it is usually a narrower shell, and always 

 differs from the published figure of that species in having its ears more dis- 

 tinct from the umbonal slopes, particularly the anterior one. Its posterior ear 

 also differs in being sinuate instead of straight, and in terminating in an acute 

 point, instead of a right angle. 



Some of our specimens are as much as one-third larger than that we have 

 figured, and slightly more oblique, with proportionally larger costae. These 

 resemble more nearly the shell figured by Prof. Cox, but always differ in having 

 the margin of the posterior ear distinctly sinuous, and its extremity acutely 

 pointed.* 



Our species also resembles Aviculopecten papyraceus, of Sowerby, but differs 

 in being less oblique, and in having a more obtuse anterior ear, and simple 

 costae, as well as a smaller and more deeply sinuous posterior ear. 



The specific name was given in honor of Prof. E. T. Cox, of the Kentucky 

 and Arkansas Geological Surveys. 



Locality and position : Adams county, Illinois, in a dark bituminous shale 

 of the Lower Coal Measures. 



Aviculopecten pellucidus, M. and "W. 



PL 26, fig. 5a, 56. 



Aviculopecten pellucidus, Meek and Worthen, Oct., 1860. Proceed. Acad. Nat. Hci., 

 Philad., p, 455. 



Shell very small, extremely thin and fragile, compressed, 

 subovate or subcircular, somewhat oblique ; hinge margin 

 straight, less than the greatest diameter of the valves from the 

 front to the posterior side ; ventral margin nearly regularly 

 rounded ; anterior edge rounded near the middle, and passing 

 by a slightly oblique curve into the ventral border ; posterior 

 margin most prominent below the middle, sloping obliquely 



* In comparing our shell with the figure and description published by Prof. Cox, it is 

 necessary that the reader should bear in mind that he has inadvertently described the 

 anterior ear as the posterior, and the posterior as the anterior. 



